The present invention relates to a projection tube and, in particular, to an electron beam lens structure for a projection tube.
Projection displays typically employ one or more projection tubes having a faceplate or screen on which an image or a sub-image to be displayed is produced, which image is projected through an optical lens system onto a viewing surface. Conventional rear-screen projection television receivers are examples of such displays. A color projection display may have a single projection tube having on its faceplate three colors of phosphors which produce a color image that is projected onto a viewing surface for viewing, or may have three projection tubes, e.g., one each for red, green and blue sub-images, which are projected and combined at the viewing surface into a color image. Conventional projection displays suffer from an inability to produce high-brightness, high-resolution imagesxe2x80x94a trade-off of improvement of one and degradation of the other must be dealt with.
The principal reasons for such lack of brightness and resolution originate with the projection tubes that initially produce the image that is projected. High brightness requires a high electron beam current impinging on the light-producing phosphors that produce the image or sub-image. High resolution requires a small spot size for the electron beam where it impinges upon the phosphors. Unfortunately, conventional projection tubes suffer from a substantially increasing spot size at high beam currents, or, in other words, an inability to provide both high beam current and small spot size at the same time. The demands being placed on better resolution and higher brightness can not be met by conventional single-beam projection tubes.
One published approach to increasing brightness involves removing the shadow mask from a conventional television color cathode ray tube (CRT) and to replace the color phosphors with a monochrome phosphor, and to simply focus the three electron beams produced by the three electron guns thereof onto the same spot on the monochrome phosphor. While an improvement in brightness has been demonstrated by such arrangement, the fundamental trade off of increased spot size vs. brightness has not been adequately solved as is required for projection tube applications. In fact, the nature of a television CRT which strives for a large screen size with minimum CRT depth dimension imposes a requirement for wide-angle beam deflection that does not enhance the small spot size/high beam current performance. In addition, the larger diameter (e.g., 29 mm) neck of a color CRT undesirably increases the cost and weight of the CRT.
Conventional wisdom suggests that greater brightness may be attainable with larger aperture CRTs, i.e. CRTs having a larger opening in the electron lens thereof The problem with a larger electron lens opening is that the CRT also requires a larger deflection yoke which consumes additional electrical power, and so generates additional heat leading to higher operating temperature and lower reliability, all of which is not desirable. The conventional alternative of extending the length of the electron gun is also undesirable because the CRT becomes too large in depth dimension as to be practically marketable. Another conventional approach is to provide additional grids within the electron lens structure biased at potentials intermediate those of the grids closer to the cathode and the screen potential, but this has the effect of increasing the effective opening of the electron lens with the same detrimental effects as described above, and still providing only a single electron beam.
Accordingly, there is a need for a projection tube having an electron lens structure that provides an improvement in both brightness and spot size over that of single-beam projection tubes and modified three-beam television-type CRTs.
To this end, the projection tube of the present invention comprises a vacuum envelope having a faceplate, a coating of phosphor on the faceplate of the projection tube, an anode electrode on the faceplate and adapted for receiving an anode potential, and an electron lens structure positioned opposite the faceplate of the projection tube for projecting a plurality N of beams of electrons toward the faceplate of the projection tube. According to a further aspect of the invention, the electron lens structure comprises a cathode including a number N of electron sources, wherein N is a positive integer greater than unity, a first electrode having N apertures therethrough axially aligned with the N electron sources for controllably passing N beams of electrons from the N electron sources toward said faceplate, a second electrode having N apertures therethrough axially aligned with the N apertures of the first electrode, each of larger diameter than and of longer axial dimension than the apertures of the first electrode, for passing the N beams of electrons from the first electrode toward and focusing the N beams of electrons substantially coincident on one spot on the faceplate, and a third electrode having N apertures therethrough axially aligned with the N apertures of the second electrode, each of larger diameter than and of longer axial dimension than the apertures of the first electrode, for passing the N beams of electrons from the second electrode to the spot on the faceplate, wherein the third electrode is adapted to be biased at a potential closer to the potential of the anode electrode with respect to the cathode potential than is the second electrode.